Polarizing glass and optical isolator

ABSTRACT

The near-field extinction ratio of a polarizing glass is increased. A polarizing glass contains anisotropically shaped metal particles oriented and dispersed in a glass substrate, which contains 0.40-0.85 wt % Cl relative to the entire glass substrate. The Vickers hardness ranges from 360 to 420, the Knoop hardness number ranges from 400 to 495, or the glass substrate contains at least one component selected from the group consisting of Y 2 O 3 , La 2 O 3 , V 2 O 3 , Ta 2 O 3 , WO 3 , and Nb 2 O 5 . The content of each of the selected components ranges from 0.05-4 mole percent, and if a plurality of the components are selected, the total content of the components is 6 mole percent or less.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a continuation of International Patent ApplicationNo. PCT/JP09/067,938, filed on Oct. 16, 2009, and claims priority toJapanese Patent Application No. 2008-267994, filed on Oct. 16, 2008, thedisclosures of which are incorporated herein by reference in theirentireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to polarizing glasses and in particular toa polarizing glass used as a polarizer in, for example, a compactoptical isolator for optical communication etc.; an optical switchcomposed of liquid crystal, an electro-optic crystal, a Faraday rotator,and so forth; or an electro-magnetic sensor.

2. Description of Related Art

It is known that a glass having anisotropically shaped fine metalparticles, such as silver particles or copper particles, oriented anddispersed therein can function as a polarizer because of its ability tochange the light absorption wavelength band of the metal particles whenthe incident polarization direction is changed.

It is also well-known that such a polarizing glass can be produced byreducing a glass containing elongated copper halide particles or a glasscontaining elongated silver halide particles. For example, JapaneseUnexamined Patent Application Publication No. 5-208844 discloses aprocess of producing a polarizing glass from a glass containing copperhalide particles. This process will be described below. First, glassserving as a matrix is made to contain Cu ions and Cl ions. Next, theglass is melted and molded. Then, the glass is cut into an appropriatesize, cast into a mold, and heat-treated. By doing so, CuClmicrocrystals with grain sizes of 50 to 300 nm are precipitated.Thereafter, the glass material in which the CuCl microcrystals have beenprecipitated is processed into a slab, which serves as a preform. At atemperature that imparts a glass viscosity in the range of 10⁷ to 10¹⁰Pa·S, the preform and the copper halide particles in the glass areelongated and then heat-treated in a reducing atmosphere. After thecopper halide particles are reduced, a polarizing glass containingelongated, anisotropically shaped metallic copper particles is produced.

The polarizing properties of such a polarizing glass are represented interms of the extinction ratio and the insertion loss defined by theExpressions below.Extinction ratio=−10 Log(□)(P///P⊥)[dB]  Expression (1)Insertion loss=−10 Log(P⊥/Pin)[dB]  Expression (2)

where Pin: intensity of incident light, P⊥: intensity of exit light inthe vertical (transmission) direction, and P//: intensity of exit lightin the parallel (extinction) direction.

When the extinction ratio is to be measured, a measuring system as shownin FIG. 1 is normally used. Light with a wavelength of 1.31 μm or 1.55μm emitted from a laser diode (LD) light source is converted via acollimating fiber into collimated light, which is incident upon aGlan-Thompson prism resulting in unidirectional linear polarization andis then incident upon a polarizing glass. Then, the polarizing glass isrotated, and the maximum and minimum amounts of light detected with apower meter are measured to calculate the extinction ratio based onExpression (1).

At this time, when the distance (measurement distance) between thepolarizing glass and the power meter is changed, the calculatedextinction ratio also changes. In short, the extinction ratio is smallfor a short measurement distance, whereas the extinction ratio is largefor a long measurement distance. This is probably because weaklyscattered light radiated from the polarizing glass in all directions isdetected by the power meter, decreasing the extinction ratio, in thecase of a short measurement distance, whereas less of such scatteredlight is detected by the power meter, increasing the extinction ratio,in the case of a long measurement distance.

This relationship between the measurement distance and the extinctionratio will be described by providing specific values.

A decrease of X (dB) in the extinction ratio of the polarizing glass, asa result of a decrease from A mm to B mm in the distance (measurementdistance) between the polarizing glass and the power meter photodetectorhaving an aperture portion, can be explained based on the schematicdiagram shown in FIG. 4. In the case of the long measurement distance A,the amount of scattered light (in terms of photo-detection area) thatcan be detected by the power meter photodetector in the case of theshort measurement distance B decreases in proportion to (B/A)*2. Incontrast, when the measurement distance decreases from A to B, theamount of scattered light that can be detected increases in proportionto (A/B)*2.

In summary, assuming that the scattered light intensity is notangle-dependent, the decrease of X (dB) in the extinction ratio of thepolarizing glass, as a result of a decrease from A mm to B mm in thedistance (measurement distance) between the polarizing glass and thepower meter photodetector with an aperture portion, can be roughlycalculated from the following Expression as a model, based on the ratiobetween the power meter photodetector areas that can detect scatteredlight.X=10 Log(A/B)*2  Expression (3)

Although the actual decrease in the extinction ratio of the polarizingglass slightly deviates from the value calculated from Expression (3)because scattered light is angle-dependent in an actual copper-basedpolarizing glass, the extinction ratio at a measurement distance of 15mm decreases to approximately 40 dB, compared with an extinction ratioof 55 dB at a measurement distance of 300 mm.

For these extinction ratio measurements of the polarizing glass,collimated light is used as the source light, with the shortest distancethat can be measured by the measuring instrument being limited to ameasurement distance of 15 mm, as shown in FIG. 1. In an optical systemfor use in an actual optical isolator, diverging light from a laserdiode (LD) light source is focused by a lens onto the optical isolator.For this reason, the proportion of re-radiated light due to resonancescattering from the polarizing glass surface being accepted by anoptical fiber decreases, compared with the optical system featuringcollimated light shown in FIG. 1. Therefore, the decrease in theextinction ratio as a result of the measurement distance being decreasedis slight. In other words, the values of the extinction ratio of thepolarizing glass and the isolation of an optical isolator incorporatingthis polarizing glass do not always match, though the relationshipbetween the extinction ratio and the isolation is such that as theextinction ratio increases, the isolation also increases.

An investigation was conducted to confirm the above-described fact.

For a copper-based polarizing glass with an extinction ratio of 40 dB ata measurement distance of 15 mm and at a wavelength of 1.55 μm, theextinction ratio measured at a measurement distance of 300 mm and at awavelength of 1.55 μm was 56 dB. When a free space optical isolator fora wavelength of 1.55 μm was assembled in the same manner as describedabove to measure its isolation, the result was 36 dB, which demonstratesthat the polarizing glass is suitable for use in the free space opticalisolator.

On the other hand, when a pigtail optical isolator for a wavelength of1.55 μm was assembled using the same sample, i.e., the copper-basedpolarizing glass with an extinction ratio of 40 dB at a measurementdistance of 15 mm and at a wavelength of 1.55 μm, together with acommercially available garnet film, permanent magnet, and single modefiber to measure the isolation of the isolator, the result was 28 dB.

Considering that the isolation of typical optical isolators is specifiedas 30 dB or more, the isolation of 28 dB was below the specification.That is, the polarizing glass with an extinction ratio of 40 dB or lessat a measurement distance of 15 mm cannot be used in a pigtail opticalisolator.

Nowadays, pigtail optical isolators in which the polarizing glass isbonded directly to the fiber are widely used as optical isolators forMetropolitan Area Networks. In such optical isolators, the near-fieldextinction ratio, defined as the extinction ratio for a short distancebetween the polarizing glass and the power meter, is important.

On the other hand, because it is presumed from the principle of Miescattering that as the minor-axis diameter of anisotropic fine metalparticles in the polarizing glass increases, this scattered light alsoincreases, it is important to produce a polarizing glass containinganisotropic fine metal particles with minimized minor-axis diameters.

From the conclusion described above, to achieve a high near-fieldextinction ratio value, i.e., a high extinction ratio value for a shortdistance between the polarizing glass and the power meter, it isadvantageous to increase the aspect ratio (major-axisdiameter/minor-axis diameter) of the post-reduced metal particles byincreasing the degree of stretching of the metal halide particles.

However, there has been a problem in that when metal halide particlesare to be stretched by heat-stretching a preform, the preform easilybreaks, decreasing the yield. This problem will be described in detail.

If the glass of the preform is hard, i.e., if the glass has a lowflexibility, then it is difficult to stretch the preform. As a result,the preform needs to be stretched with a high tensile force to increasethe degree of stretching of the metal halide fine particles. It is truethat the rate of stretching of metal halide fine particles increaseswith a high tensile force, but the probability of the preform beingbroken due to that high tensile force also increases.

On the other hand, if the glass of the preform is soft, i.e., if theglass has a high flexibility, the preform can be stretched with a lowtensile force. However, because such a glass has a low mechanicalstrength and is fragile, the glass may break even with a low tensileforce.

Thus, because the required tensile force cannot be applied to the metalhalide fine particles, whether the preform glass is hard or soft, in anattempt to stretch the preform with a tensile force such as not to causeglass breakage, it is difficult to increase the degree of stretching ofthe metal halide fine particles. Therefore, it is not possible toincrease the near-field extinction ratio, as described below in detail.

To achieve a high extinction ratio, a tensile force that is almost ashigh as that at which the preform glass breaks is normally applied tostretch the preform in the stretching process. However, the rate ofstretching of CuCl will not surpass a certain level, whether the preformglass is hard or soft, under conditions in which frequent breakage mustbe avoided in the production process. Consequently, the rate ofstretching of the metal halide particles has not been enhanced due to anincrease in the probability of fracture.

As described so far, when a known copper-based polarizing glass is to bestretched with a high tensile force in an attempt to increase the rateof stretching, i.e., the aspect ratio (major-axis diameter/minor-axisdiameter), of metal halide fine particles in the form of precursors ofanisotropic fine metal particles, there is a possibility of the glassbeing broken during the stretching process. For this reason, theminor-axis diameters of the anisotropic metal particles cannot bedecreased if a high yield is to be achieved in the production process.Therefore, the extinction ratio at a measurement distance of 15 mm is aslow as about 40 dB. Consequently, there has been a problem in that theisolation of a pigtail optical isolator incorporating a conventionalpolarizing glass is too low to satisfy an isolation performance of 30 dBor more, which is the specification of typical isolators, even thoughthe same polarizing glass can be satisfactorily used in a free spaceoptical isolator.

BRIEF SUMMARY OF THE INVENTION

A first aspect according to the present invention is a polarizing glassincluding anisotropically shaped copper particles oriented and dispersedin a glass substrate, wherein raw materials of the glass substrate arecomposed of, in terms of wt %:

SiO₂: 48-65,

B₂O₃: 13-33,

Al₂O₃: 6-13,

AlF₃: 0-5,

alkali metal oxide: 7-17,

alkali metal chloride: 0-5,

alkali earth metal oxide: 0-5,

copper oxide and copper halide: 0.3-2.5,

SnO: 0-0.6, and

As₂O₃: 0-5;

the Cl content in the glass substrate ranges from 0.40 to 0.85 wt %relative to the entire glass substrate; and the Vickers hardness of theglass substrate ranges from 360 to 420.

A second aspect according to the present invention is a polarizing glassincluding anisotropically shaped copper particles oriented and dispersedin a glass substrate, wherein the glass substrate is composed of, interms of wt % relative to the entire glass substrate:

SiO₂: 55-63,

B₂O₃: 16-22,

Al₂O₃: 7-10,

alkali metal oxide: 8-12,

alkali earth metal oxide: 0-3,

copper oxide: 0.3-1.0,

SnO: 0-0.3,

As₂O₃: 0-3,

F: 0.5-1.0, and

Cl: 0.40-0.85;

and the Vickers hardness of the glass substrate ranges from 360 to 420.

A third aspect according to the present invention is a polarizing glassincluding anisotropically shaped copper particles oriented and dispersedin a glass substrate, wherein raw materials of the glass substrate arecomposed of, in terms of wt %:

SiO₂: 48-65,

B₂O₃: 13-33,

Al₂O₃: 6-13,

AlF₃: 0-5,

alkali metal oxide: 7-17,

alkali metal chloride: 0-5,

alkali earth metal oxide: 0-5,

copper oxide and copper halide: 0.3-2.5,

SnO: 0-0.6, and

As₂O₃: 0-5;

the Cl content in the glass substrate ranges from 0.40 to 0.85 wt %relative to the entire glass substrate; and the Knoop hardness number ofthe glass substrate ranges from 400 to 495.

A fourth aspect according to the present invention is a polarizing glassincluding anisotropically shaped copper particles oriented and dispersedin a glass substrate, wherein the glass substrate is composed of, interms of wt % relative to the entire glass substrate:

SiO₂: 55-63,

B₂O₃: 16-22,

Al₂O₃: 7-10,

alkali metal oxide: 8-12,

alkali earth metal oxide: 0-3,

copper oxide: 0.3-1.0,

SnO: 0-0.3,

As₂O₃: 0-3,

F: 0.5-1.0, and

Cl: 0.40-0.85;

and the Knoop hardness number of the glass substrate ranges from 400 to495.

A fifth aspect according to the present invention is a polarizing glassincluding anisotropically shaped metal particles oriented and dispersedin a glass substrate, wherein the glass substrate is glass selected fromthe group consisting of silicate glass, borate glass, and borosilicateglass; the glass substrate contains at least one component selected fromthe group consisting of Y₂O₃, La₂O₃, V₂O₃, Ta₂O₃, WO₃, and Nb₂O₅; thecontent of each of the selected components ranges from 0.05 to 4 molepercent, or the total content of the components is 6 mole percent orless if a plurality of components are selected; and the Cl content inthe glass substrate ranges from 0.40 to 0.85 wt % relative to the entireglass substrate.

A sixth aspect according to the present invention is characterized inthat, in any of the first to fourth aspects, the glass substrate is aglass selected from the group consisting of silicate glass, borateglass, and borosilicate glass; the glass substrate contains at least onecomponent selected from the group consisting of Y₂O₃, La₂O₃, V₂O₃,Ta₂O₃, WO₃, and Nb₂O₅; and the content of each of the selectedcomponents ranges from 0.05 to 4 mole percent, or the total content ofthe components is 6 mole percent or less if a plurality of componentsare selected.

A seventh aspect according to the present invention is characterized inthat, in any of the first to sixth aspects, an extinction ratio at ameasurement distance of 15 mm is 44 dB or more in response to at leastone of light in a wavelength band with a center wavelength of 1.31 μmand light in a wavelength band with a center wavelength of 1.55 μm.

An eight aspect according to the present invention is characterized inthat, in any of the first to seventh aspects, the maximum of theabsolute values of the amounts of shift in a polarization axis at sites5 mm away from a center point on a principle surface is 0.15° or less.

A ninth aspect according to the present invention is an optical isolatorincluding the polarizing glass described in any of the first to eighthaspects.

A tenth aspect according to the present invention is an optical isolatorincluding a Faraday rotating element and at least one polarizer, whereinthe polarizer includes the polarizing glass described in any of thefirst to ninth aspects.

An eleventh aspect according to the present invention is a process ofproducing a polarizing glass containing anisotropically shaped metalparticles oriented and dispersed in a glass substrate by stretching aglass preform, the process including melting raw materials of the glasssubstrate in an airtight melting furnace; and replacing at least one ofthe alkali oxide components in the raw materials of the glass substratewith an alkali chloride component, wherein the Cl content in the glasssubstrate ranges from 0.40 to 0.85 wt % relative to the entire glasssubstrate.

A twelfth aspect according to the present invention is related to theeleventh aspect and characterized in that the anisotropically shapedmetal particles are copper particles; raw materials of the glasssubstrate are melted, the raw materials being composed of, in terms ofwt %:

SiO₂: 48-65,

B₂O₃: 13-33,

Al₂O₃: 6-13,

AlF₃: 0-5,

alkali metal oxide: 7-17,

alkali metal chloride: 0-5,

alkali earth metal oxide: 0-5,

copper oxide and copper halide: 0.3-2.5,

SnO: 0-0.6, and

As₂O₃: 0-5;

and the Vickers hardness of the glass substrate ranges from 360 to 420.

A thirteenth aspect according to the present invention is related to theeleventh aspect and characterized in that the anisotropically shapedmetal particles are copper particles; raw materials of the glasssubstrate are melted, the raw materials being composed of, in terms ofwt %:

SiO₂: 48-65,

B₂O₃: 13-33,

Al₂O₃: 6-13,

AlF₃: 0-5,

alkali metal oxide: 7-17,

alkali metal chloride: 0-5,

alkali earth metal oxide: 0-5,

copper oxide and copper halide: 0.3-2.5,

SnO: 0-0.6, and

As₂O₃: 0-5;

and the Knoop hardness number of the glass substrate ranges from 400 to495.

According to the present invention, metal halide fine particles can bemade relatively stretchable, and the minor-axis of the anisotropic finemetal particles can be reduced by providing a relatively stretchableglass serving as a matrix, thereby providing a polarizing glass with ahigh near-field extinction ratio at a short measurement distance.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 illustrates measurement of an extinction ratio.

FIG. 2 shows the structure of a glass serving as a matrix according tothe present invention.

FIG. 3 shows the structure of a pigtail optical isolator.

FIG. 4 illustrates measurement distances and the ratio betweenphoto-detection areas.

FIG. 5 illustrates the relationship between the measurement distance andthe extinction ratio.

FIG. 6 illustrates the relationship between the amount of Cl in a glasssubstrate, and the Vickers hardness and the Knoop hardness number.

FIG. 7 illustrates the relationship between the amount of Cl in a glasssubstrate and the near-field extinction ratio.

DETAILED DESCRIPTION OF THE INVENTION

To produce a polarizing glass with a high near-field extinction ratio ata high yield while still preventing the preform from breaking when it isstretched with a tensile force high enough to increase the aspect ratiosof metal halide fine particles, the preform needs to endure that tensileforce so as not to break. In the current description, the nature of theglass of a preform capable of being stretched with a higher tensileforce without breaking is referred to as “easily stretchable” or“superior in the ease of stretching.”

How easily a preform glass can be stretched depends on the strength ofthe glass structure (referred to as the glass framework structure).

For example, a glass with a strong glass structure does not easilystretch, and will break if an attempt is made to stretch the glass witha high tensile force.

On the other hand, although a glass with a weak glass structure easilystretches with a low tensile force, such a glass easily breaks even witha lower tensile force due to the weak glass structure, and therefore, itis not possible to apply an appropriately high tensile force when theglass is to be stretched. For this reason, the aspect ratios of themetal halide fine particles cannot be increased.

This means that it is difficult to stretch a glass, whether it has astrong glass structure or a weak glass structure. In other words, such aglass has low ease of stretching. To produce a polarizing glass with ahigh near-field extinction ratio, i.e., to achieve an object of thepresent invention, it is necessary to increase the ease of stretching ofthe glass serving as a matrix.

The following two factors can be considered to control the strength ofthe glass structure as to the ease of stretching of the glass serving asa matrix.

(1) Temperature at the Time of Stretching

The glass structure is weak at high temperature, whereas the glassstructure is strong at low temperature.

(2) The Amount of Alkali Metal Oxides, Boric Acids, Phosphoric Acids,and so Forth in the Glass Substrate

The glass structure becomes weaker with a high concentration of alkalimetal oxides, boric acids, phosphoric acids, and so forth in the glasssubstrate, whereas the glass structure becomes stronger with a lowerconcentration of those substances.

With the composition of a known polarizing glass, it was not possible toapply a high tensile force to the preform because it would break due tothe tensile force, whether the temperature at the time of stretching washigh or low, thus inhibiting the aspect ratios of the metal halide fineparticles from increasing.

On the other hand, means for adjusting the component amounts of alkalimetal oxides, boric acids, phosphoric acids, and so forth, based on thecomposition of a known polarizing glass, was problematic in that “thechemical durability of the glass may decrease,” “the glass may becrystallized,” “crystallization of the metal halide fine particles maybe adversely affected (crystallization is inhibited, large crystals areprecipitated),” and so forth.

In a situation where such problems exist, the inventors have madeintensive efforts to find means for enhancing the ease of stretching ofa preform glass. As a result, the inventors have found that the strengthof the glass structure greatly changes depending on the amount of Clcontained in the glass serving as a matrix. More specifically, theinventors have found that the glass structure becomes stronger as theamount of Cl decreases, increasing the Vickers hardness and the Knoophardness number of the glass serving as a matrix, whereas the glassstructure becomes weaker as the amount of Cl in the glass serving as amatrix increases. Furthermore, the inventors have found that the Vickershardness or the Knoop hardness number of the glass serving as a matrixdecreases with an increase in the amount of Cl in the glass serving as amatrix. The inventors have found that as a result of a decrease in theVickers hardness or the Knoop hardness number, the near-field extinctionratio of the polarizing glass obtained from the preform glass can beincreased.

This embodiment will now be described.

To produce a polarizing glass with a high extinction ratio, i.e., toachieve an object of the present invention, the raw materials of thepolarizing glass should preferably be composed of SiO₂: 48-65, B₂O₃:13-33, Al₂O₃: 6-13, AlF₃: 0-5, alkali metal oxides: 7-17, alkali metalchlorides: 0-5, alkali earth metal oxides: 0-5, copper oxides and copperhalides: 0.3-2.5, SnO: 0-0.6, and As₂O₃: 0-5 on a wt % basis in such amanner that the composition constitutes 100 wt % in total.

For the above-described individual components, the following substancesare used, for example.

SiO₂: SiO₂ etc.

B₂O₃: H₃BO₃, B₂O₃, etc.

Al₂O₃: Al (OH)₃, Al₂O₃, etc.

Alkali metal oxides: carbonates, nitrates, chlorides, fluorides, etc.

Alkali earth metal oxides: carbonates, nitrates, chlorides, fluorides,etc.

Copper oxide: oxide, chloride, fluoride, nitrate, etc.

SnO: SnO, SnO₂, etc.

As₂O₃: As₂O₃ etc.

F: cationic fluorides of the above-described oxide components (e.g.,AlF₃)

Cl: cationic chlorides of the above-described oxide components (e.g.,NaCl)

In this embodiment, the amount of Cl contained in this glass substrateaccounts for 0.40 to 0.85 wt % relative to the entire glass substrateconstituting the polarizing glass. Although described later in detail,with the amount of Cl in the glass substrate falling within theabove-described range, the aspect ratio of the metal halide fineparticles can be increased by increasing the percentage of weak bonds toall bonds in the glass when it is stretched with such a tensile force asnot to cause breakage, thereby enhancing the near-field extinction ratioof the glass.

In this case, means for introducing Cl into the glass serving as amatrix includes those listed below. Any of these means can be employed.

Means 1: Replacing some of raw materials (Na₂CO₃, KNO₃, etc.) of analkali oxide component (e.g., Na₂O or K₂O) with a raw materialcontaining an alkali element and Cl (e.g., NaCl or KCl).

Means 2: Adding a raw material containing Cl, such as NH₄Cl, HClO, orHClO₃.

Means 3: Spraying the glass melt with a gas containing Cl (e.g., Cl₂ orSOCl₂) while the glass serving as a matrix is being melted.

Means 4: Bubbling the glass melt using a gas containing Cl (e.g., Cl₂ orSOCl₂) while the glass serving as a matrix is being melted.

It should be noted, however, that because Cl-containing gases areextremely toxic and need to be handled with extreme care, use ofCl-containing gases is not preferable. Furthermore, if means forincreasing the percentage of raw materials containing Cl (e.g.,increasing the amount of replacement from NaCO₃ to NaCl) or means foradding a large amount of Cl sources (e.g., NH₄Cl) were employed becauseCl easily volatilizes during glass melting and hardly remains in theglass, gases containing highly toxic Cl would be emitted from themelting furnace into the atmosphere, which would require an extensivetoxic substance elimination facility. For this reason, these means arenot preferable, either.

To address these difficulties, this embodiment has made it possible, bysatisfying any of the following conditions, to increase the Clconcentration in the glass without having to use gases containing highlytoxic Cl or without having to increase the Cl concentration in the glassraw materials.

Condition 1: Adding at least one component selected from the groupconsisting of Y₂O₃, La₂O₃, V₂O₅, Ta₂O₅, WO₃, and Nb₂O₅ to the componentsconstituting the glass.

Condition 2: Melting the glass in a highly airtight melting furnace.

Under Condition 1, when at least one component selected from among Y₂O₃,La₂O₃, V₂O₅, Ta₂O₅, WO₃, and Nb₂O₅ is added to the glass composition,the Cl concentration in the glass can be increased, compared with thecomposition to which none of these components are added. Although themechanism of increase of the amount of Cl remaining in the glass is notclear, it is presumed that adding at least one component selected fromamong Y₂O₃, La₂O₃, V₂O₅, Ta₂O₅, WO₃, and Nb₂O₅ into the glass preventsthe glass melting temperature from increasing significantly, increasesthe viscosity of the glass melt, and suppresses volatilization of Clduring melting, thus increasing the amount of remaining Cl.

When at least one of Y₂O₃, La₂O₃, V₂O₅, Ta₂O₅, WO₃, and Nb₂O₅ is addedto the glass in this manner, Cl easily remains in the glass. In theglass substrate after having become a glass, the content of one of thesecomponents should preferably fall within 0.05 to 4% in terms of mol %,and the total content of two or more of these components shouldpreferably be below 6% in terms of mol %. When the contents of the addedcomponents are below 0.05%, the effect of causing Cl to remain in theglass is small. On the other hand, when the content of an component isabove 4% or the total content of two or more components are above 6%,problems, such as the glass being easily crystallized and the thermalcharacteristic being increased (the stretching temperature beingincreased), are raised.

Although oxides (Y₂O₃, La₂O₃, V₂O₅, Ta₂O₅, WO₃, and Nb₂O₅) are mainlyused for the raw materials of these added components, some or all ofthese oxides may be replaced with fluorides or chlorides.

On the other hand, the term “highly airtight melting furnace” in thecontext of Condition 2 refers to a melting furnace that has been mademore airtight by, for example, not providing a front door through whicha batch of raw materials is delivered or a test glass is collected. Witha highly airtight melting furnace, the Cl concentration in theatmosphere surrounding a crucible filled with a glass melt ismaintained. As a result, Cl is prevented from volatilizing from theglass melt, thereby increasing the amount of Cl in the glass. A meltingfurnace without a front door is just one example of a highly airtightmelting furnace. A highly airtight melting furnace generally refers to afurnace having only a few gaps through which the atmosphere in themelting furnace leaks outside. Furthermore, with a highly airtightmelting furnace, the airtightness of the furnace can be increased,whether it is a batch melting furnace or a continuous melting furnace.As a result, the amount of Cl in the glass can be increased. For a batchmelting furnace, the Cl concentration in the atmosphere interfacing withthe glass melt can also be maintained by placing a lid over the pot inwhich the glass serving as a matrix is melted, which brings about thesame effect as when a highly airtight melting furnace is used.

As described above, when a glass substrate that will become a polarizingglass is to be produced from glass raw materials, the glass substrateafter having become a glass should preferably be composed of SiO₂:55-63, B₂O₃: 16-22, Al₂O₃: 7-10, alkali metal oxide: 8-12, alkali earthmetal oxide: 0-3, copper oxide: 0.3-1.0, SnO: 0-0.3, As₂O₃: 0-3, F:0.5-1.0, and Cl: 0.40-0.85, in terms of wt %, in such a manner that thecomposition constitutes 100 wt % in total.

As described above, in this embodiment, when a glass substrate that willbecome a polarizing glass is to be produced, the composition of theglass substrate after having become a glass contains 0.40 to 0.85 wt %of Cl. The reason for this will be described below in detail.

First, the structure of an oxide glass will be described.

In an oxide glass containing SiO₂, it is presumed that tetrahedrons eachcomposed of an Si atom and four oxygens surrounding the Si are bondedwith bridging oxygens such that these tetrahedrons spreadthree-dimensionally in a net-like manner. As the amount of Cl in theglass having the glass structure shown in FIG. 2( a) increases, Cl atomsbreak into the positions of some bridging oxygens, changing the strongcovalent bonding of the bridging oxygens to the weak ionic binding ofCl, as shown in FIG. 2( b). As a result, partially weak bonding occursin the glass structure, which thus becomes weak. In contrast, when theamount of Cl in the glass decreases, the number of partially weak bondsdecreases, making the glass structure strong.

As shown below (Source: HANDBOOK of CHEMISTRY and PHYSICS 79^(TH)EDITION etc.), Cl ions have larger diameters than cations, such as Si,and anions, such as O, used in a polarizing glass, and even a slightchange in the amount of Cl greatly affects the glass structure.

Si: 0.26-0.40 Å

B: 0.20 Å

Al: 0.39-0.54 Å

Na: 0.99-1.24 Å

O: 1.40 Å

F: 1.36 Å

Cl: 1.81 Å

For a specific example of the above-described theory, how the glassstructure changes as a result of introduction of Cl into the glass canbe explained by way of analogy of a rubber band. Just as an old rubberband easily stretches, a structure containing partially weak bonding(refer to FIG. 2( b)) features high ease of stretching in response to atensile force. On the other hand, in the same manner as a very oldrubber band containing an extremely large amount of weak bonding easilybreaks with a weak force, a glass containing increased weak bonding(i.e., a large amount of Cl) breaks with a weak tensile force.

The amount of partially weak bonding in the glass, that is, the strengthof the glass structure, is reflected in the hardness of the glass. TheVickers hardness and the Knoop hardness number of a glass with a strongglass structure (small amount of partially weak bonding) have a tendencyto become high, whereas the Vickers hardness and the Knoop hardnessnumber of a glass with a weak glass structure (large amount of partiallyweak bonding) have a tendency to become low.

In this embodiment, the glass melting conditions and the composition ofdelivery raw materials are adjusted so that the amount of Cl containedin the glass serving as a matrix falls within a range of 0.40 wt % to0.85 wt %. By doing so, the Vickers hardness of the glass serving as amatrix is set at 360 to 420, or the Knoop hardness number of the sameglass is set at 400 to 495. As a result, the near-field extinction ratiomeasured at a measurement distance of 15 mm is increased to 44 dB ormore.

In this case, if the amount of Cl in the glass is below 0.40 wt %, theabove-described effect of increasing the aspect ratio due to softeningof the glass preform cannot be brought about.

On the other hand, if the amount of Cl in the glass is above 0.85 wt %,the percentage of weak bonds to all bonds in the glass increases, makingthe glass structure extremely weak. In this case, the preform is solikely to break at the time of stretching that if the preform isstretched with such a tensile force as not cause breakage, the aspectratio of metal halide fine particles will not increase, and what is evenworse, the near-field extinction ratio may decrease in some cases.

Here, the relationship between the amount of Cl in the glass and thenear-field extinction ratio measured at a measurement distance of 15 mmis shown in FIG. 7, and the relationship between the amount of Cl in theglass and the hardness (Vickers hardness and Knoop hardness number) ofthe glass is shown in FIG. 6. These figures show that if the amount ofCl in the glass ranges from 0.40 wt % to 0.85 wt %, the Vickers hardnessis from 360 to 420 (FIG. 6), the Knoop hardness number is from 400 to495 (FIG. 6), and the near-field extinction ratio measured at ameasurement distance of 15 mm is 44 dB or more (FIG. 7).

Both the Vickers hardness and the Knoop hardness number are indices ofhardness with respect to a certain reference and do not usually have aunit. It is also known that both the Vickers hardness and the Knoophardness number vary in measurements. For this reason, in the presentembodiment and Examples, measurement is carried out assuming that therecords of measured specimens (particularly, physical states such asresidual stress) are the same. Furthermore, measurement is carried outmore than once for the same specimen, or more than one specimen isproduced for the same glass if necessary, to average out the measurementvariations when physical properties are compared.

Next, the reason that when a glass substrate that will become apolarizing glass is to be produced, Cl should account for 0.40 to 0.85wt % of the composition of the glass substrate after having become aglass will be described from the viewpoint of oxide glass structure, aswell as from the viewpoint of the grain size of metal particles.

As described above, when the amount of Cl remaining in the glass is 0.40wt % or more, the Vickers hardness of the glass serving as a matrix is420 or less, or the Knoop hardness number is 495 or less. Therefore, thenear-field extinction ratio of a polarizing glass produced from thisglass serving as a matrix increases to 44 dB or more when measured at ameasurement distance of 15 mm.

In this embodiment, CuCl fine particles or AgCl fine particles with agrain size of 50 to 300 nm are precipitated by heat-treating atransparent glass, serving as a matrix, containing Cl ions and either ofCu ions and Ag ions. On the other hand, when the amount of Cl in theglass exceeds 0.85 wt %, CuCl crystals or AgCl crystals with large grainsizes are precipitated as soon as the glass serving as a matrix isproduced, readily producing a white turbid glass. In such a glass, itbecomes difficult to control the grain sizes of CuCl or AgCl crystals tobe precipitated by heat treatment, and crystals with large grain sizesare generated in a certain percentage.

As a result, a polarizing glass with high insertion loss and lowperformance is produced.

As described above, when the amount of Cl in the glass serving as amatrix exceeds 0.85 wt %, the glass is not suitable as a matrix for apolarizing glass with a high near-field extinction ratio. The hardnessof a glass containing more than 0.85 wt % of Cl becomes much lowerbecause of an increase in weak bonds in the glass. The Vickers hardnessand the Knoop hardness number of a glass containing more than 0.85 wt %of Cl are less than 360 and less than 400, respectively. Whether a glassserving as a matrix is suitable or not can be determined by measuringits hardness.

In this embodiment, the polarizing glass produced in this manner is alsoused as an optical isolator, which will be described below. In a processin which two sheets of polarizing glass are bonded so as to sandwich agarnet film between them and are cut into a small size, the problem of ashift in the polarization axis on the polarizing glass surface occurs.Here, with respect to the reference angle at which the highestextinction ratio can be obtained at the center position on the surfaceof a polarizing glass about 10 to 15 mm square, the absolute value ofthe difference between the reference angle and the angle that gives thehighest extinction ratio at a site away from the center position isreferred to as the shift in the polarization axis. In a process in whichtwo sheets of polarizing glass are bonded so as to sandwich a garnetfilm between them and cut into a square of about 0.5 mm as describedabove, the two sheets of bonded polarizing glass are bonded shifted by45° at the perimeters of the polarizing glass if the shift in thepolarization axis is large on the polarizing glass surface. As a result,when the optical isolator is assembled, an appropriate isolation cannotbe achieved.

Here, generation of a shift in the polarization axis of the polarizingglasses is regarded as a type of shape regression occurring duringpreform drawing. The polarization axes, significantly deformed to openin a V shape, are considered to open in a significantly small inverted-Vshape just before the structure is frozen to a predetermined width inthe drawing process.

A glass serving as a matrix, according to this embodiment, whosemechanical strength is relatively low because of 0.40 to 0.85 wt % of Clcontained in the glass substrate has a partially weak structure. Forthis reason, the resilience for restoring the polarization axes isconsidered to be small at the microscopic level.

In fact, the inventors of the present invention have confirmed that whenthe Vickers hardness of the glass serving as a matrix is 420 or less orthe Knoop hardness number is 495 or less, the absolute value of themaximum of the shifts in the polarization axis at points 5 mm away fromthe center position on the main surface of the polarizing glassdecreases to 0.15° or less. However, because a glass with a Vickershardness of less than 360 or a Knoop hardness number of less than 400breaks or exhibits a low near-field extinction ratio as described above,the Vickers hardness of the glass serving as a matrix needs to be from360 to 420 and the Knoop hardness number from 400 to 495.

EXAMPLES

The present invention will now be described by way of Examples. Itshould be noted, however, that the present invention is not limited bythe Examples described below.

Before Examples and Comparative Examples are described, the meaning ofthe composition of raw materials in Tables 1, 2, and 5 showing thecomposition of raw materials of the glass substrate is described.

In Tables 1, 2, and 5, the values for the metal chlorides (NaCl and CuClin Tables 1 and 2 and AgCl in Table 5) and the metal fluoride (AlF₃ inTable 1) used as a Cl source and an F source represent their contentpercentages in the form of raw materials. On the other hand, the valuesfor the other oxides represent their content percentages not in the formof raw materials used but in the form of oxides produced from the rawmaterials used. For example, “Na₂O” does not mean used raw materials butoxides produced from Na₂CO₃ or NaNO₃ used as raw materials. In addition,the values for “Na₂O” mean the content percentages in the form of Na₂Oproduced from those raw materials. When Cl and F are to be introducedinto a glass, metal chlorides and metal fluorides are normally used asraw materials, but Cl and F do not readily remain in the glass becausethey easily volatilize during melting. This is why the contentpercentages in the form of raw materials were employed.

In addition, when a minor component is contained in the glass, like acolored glass used as a colored glass filter or a polarizing glassaccording to the present invention, the minor component is usually addedto the original glass in mass ratio for the composition of rawmaterials. For the composition of raw materials according to the presentinvention, CuCl, AgCl, and SnO are minor components and are representedin mass ratio.

Examples in a Case where Cu Particles are Contained

First, Examples of a glass containing Cu particles are described.Glasses with the composition described (in terms of weight %) in ExampleNos. 1 to 3, 6, and 7 of Table 1 were produced as described below. Rawmaterials were delivered in nine batches in three hours at 1350° C. intoa 5-liter platinum crucible placed in a cubic, highly hermetic800×800×800 mm-capacity melting furnace without a front door.Subsequently, the raw materials were left at rest for 30 minutes andmelted. Thereafter, the temperature was increased to 1410° C., and theraw materials were left at rest for 30 minutes. Then, the raw materialswere stirred with a crank-type stirring rod at a rotational speed of 80rpm for 30 minutes and left at rest for 30 minutes. This was repeatedtwice. Then, after stirring for a third time for 30 minutes, the rawmaterials were left at rest for two hours to remove air bubbles in theglass melt, and the temperature was decreased to 1390° C. in 30 minutes.Then, the raw materials were subjected to finish stirring for 90 minutesat a rotational speed of 50 rpm, poured into a casting mold, and cooledslowly at 490° C. to produce a glass block.

TABLE 1 Example No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 No. 7 No. 8 Rawmaterial SiO₂ 59.87 59.87 59.87 58.84 57.81 59.87 59.87 58.84composition AlF₃ 2.04 2.04 2.04 2.00 1.97 2.04 2.04 2.00 in glass Al₂O₃6.82 6.82 6.82 6.70 6.59 6.82 6.82 6.71 substrate/ B₂O₃ 20.54 20.5420.54 20.18 19.84 20.54 20.54 20.18 wt % NaCl 1.23 1.36 1.70 1.67 1.642.04 2.35 2.31 Na₂O 9.50 9.37 9.03 8.87 8.72 8.69 8.38 8.23 Y₂O₃ 1.743.43 1.73 (0.50) (1.00) (0.50) La₂O₃ V₂O₅ Ta₂O₅ WO₃ Nb₂O₅ Subtotal100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 CuCl 0.52030.5203 0.5203 0.5203 0.5203 0.5203 0.5203 0.5203 SnO 0.0913 0.09130.0913 0.0913 0.0913 0.0913 0.0913 0.0913 Total 100.6116 100.6116100.6116 100.6116 100.6116 100.6116 100.6116 100.6116 Cl content/wt %0.40 0.46 0.49 0.53 0.56 0.43 0.42 0.47 Vickers Avg. 420 418 408 405 403411 415 414 Hv Max. 427 428 415 413 412 427 423 423 Min. 413 400 401 398394 400 409 403 Knoop Avg. 495 482 451 455 459 470 467 453 Hk Max. 501493 459 465 468 479 476 464 Min. 482 472 446 449 450 462 457 441Near-field extinction 44.0 45.5 45.7 45.9 46.1 45.1 44.8 45.5 ratio*/dBShift in polarization 0.150 0.136 0.129 0.121 0.108 0.136 0.142 0.131axis/° Note: Values in parentheses in Y₂O₃, La₂O₃, Ce₂O₃, V₂O₅, Ta₂O₅,WO₃, and Nb₂O₅ fields indicate contents in terms of mol %. Near-fieldextinction ratios indicate values measured at a measurement distance of15 mm.

TABLE 2 Comparative Example Example No. 9 No. 10 No. 11 No. 12 No. 13No. 1 No. 2 Raw material SiO₂ 58.39 59.04 57.88 58.81 58.66 59.87 56.39composition AlF₃ 1.98 2.01 1.97 2.00 1.99 2.04 2.5 in glass Al₂O₃ 6.656.72 6.59 6.70 6.68 6.82 9.05 substrate/ B₂O₃ 20.03 20.25 19.85 20.1720.12 20.54 20.36 wt % NaCl 1.66 1.68 1.64 1.67 1.66 1.23 7.22 Na₂O 8.808.90 8.73 8.87 8.84 9.50 4.48 Y₂O₃ La₂O₃ 2.49 (0.50) V₂O₅ 1.40 (0.50)Ta₂O₅ 3.34 (0.50) WO₃ 1.78 (0.50) Nb₂O₅ 2.04 (0.50) Subtotal 100.00100.00 100.00 100.00 100.00 100.00 100.00 CuCl 0.5203 0.5203 0.52030.5203 0.5203 0.5203 1.096 SnO 0.0913 0.0913 0.0913 0.0913 0.0913 0.09130.0913 Total 100.61 100.6116 100.6116 100.6116 100.6116 100.6116101.1873 Cl content/wt % 0.51 0.52 0.51 0.52 0.52 0.34 1.14 Vickers Avg.408 410 405 406 408 428 325 Hv Max. 414 417 416 415 416 439 333 Min. 402402 399 400 402 415 318 Knoop Avg. 465 470 465 470 463 504 365 Hk Max.472 480 473 478 470 509 372 Min. 458 463 458 463 456 497 356 Near-fieldextinction 45.7 45.8 45.7 45.9 45.7 40.5 37.5 ratio*/dB Shift inpolarization 0.125 0.124 0.124 0.123 0.123 0.182 0.153 axis/° Note:Values in parentheses in Y₂O₃, La₂O₃, V₂O₅, Ta₂O₅, WO₃, and Nb₂O₅ fieldsindicate contents in terms of mol %. Near-field extinction ratiosindicate values measured at a measurement distance of 15 mm.

Glasses with the composition according to Example Nos. 4, 5, and 8 inTable 1 and Example Nos. 9 to 13 in Table 2 are examples of glassescontaining components Y₂O₃, La₂O₃, V₂O₅, Ta₂O₅, WO₃, and Nb₂O₅. Theseglasses were produced as glass blocks in the same manner as Example Nos.1 to 3, 6, and 7, except that the melting temperature was 20° C. higherthan that of Example No. 1 etc. in all schedules and the slow coolingtemperature was 505° C.

From each of these glass blocks, a measuring specimen 15.0 mm indiameter and 7 mm in thickness, with one surface optically polished andthe other polished using #1000 abrasive grains to allow the opticallypolished surface to be perceived easily, was produced. The specimen washeld for two hours at the Tg (glass-transition point)±5° C. andheat-treated at a rate of temperature decrease of 10° C./hour to removethe residual stress in the processed glass specimen. Thereafter, theVickers hardness was measured using the Vickers hardness tester VMT-7Smanufactured by Matsuzawa Co., Ltd. in accordance with the JIS Z 2244Vickers hardness testing method.

For measurement, a four-sided pyramid indenter with an angle between theopposite faces of 136° was pressed onto the optically polished surfaceof the specimen at a load F (2.942 N in the present invention) to make adent in the surface, the lengths of the two diagonal lines of thepermanent dent were measured, and then the average length L (mm) wascalculated to obtain the Vickers hardness from the Expression below.Vickers hardness HV=0.1891×(F/L2)  Expression (4)

Furthermore, the Knoop hardness number of a measuring specimen that wasprocessed into the same shape and annealed in the same way to remove theresidual stress was measured using the hardness testing instrumentMVK-EII manufactured by AKASHI in accordance with the JIS Z 2251 Knoophardness number testing method. For Knoop hardness number measurement, afour-sided pyramid diamond indenter, rhombic in cross section, withangles between the two opposite sides of 172°30′ and 130° was pressedonto the optically polished surface of the specimen at a load F (0.9807N in the present invention) for 15 seconds to make a dent, and thelarger diagonal line length L (mm) of the permanent dent was measured tocalculate the Knoop hardness number from the Expression below.Knoop hardness number Hk=1.451×(F/L2)  Expression (5)

The Vickers hardness and the Knoop hardness number were measured 20times each. The mean values are shown in Tables 1 and 2. The maximum andminimum Vickers hardnesses and the maximum and minimum Knoop hardnessnumbers of the measurements are also shown in Tables 1 and 2.

The components (Si, Al, B, Na, Cu, Sn, F, Cl, Y, La, V, Ta, W, and Nb)of the glasses produced according to Example Nos. 1 to 13 werequantitatively analyzed by Sumika Chemical Analysis Service Ltd. Basedon the obtained results, the percentages of Si, Al, B, Na, Cu, Sn, Y,La, V, Ta, W, Nb, F, and Cl in the glass substrate were obtained,assuming that Si, Al, B, Na, Cu, Sn, Y, La, V, Ta, W, and Nb are presentin the form of oxides (SiO₂, Al₂O₃, B₂O₃, Na₂O, CuO, SnO, Y₂O₃, La₂O₃,V₂O₅, Ta₂O₅, WO₃, and Nb₂O₅) and that F and Cl are present in the formof F and Cl in the glass substrate. The results are shown in Tables 3and 4. To measure the Cl content, the glass specimen was broken intopieces, which were then dissolved by pyrophosphoric acid andsteam-distilled. Then, the Cl content was measured using an ionchromatographic apparatus (ICS-1000 manufactured by Dionex Corporation).

FIG. 6 shows the relationship between the amount of Cl in the glasssubstrate; and the Vickers hardness and the Knoop hardness number. Thisfigure demonstrates that as the amount of Cl in the glass substrateincreases, the Vickers hardness and the Knoop hardness number decrease.

TABLE 3 Example No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 No. 7 No. 8 Rawmaterial SiO₂ 61.17 61.14 61.13 59.94 58.79 61.16 61.21 59.97composition Al₂O₃ 8.11 8.12 8.11 7.96 7.84 8.10 8.11 7.97 in glass B₂O₃19.30 19.28 19.26 18.92 18.60 19.29 19.27 18.91 substrate/ Na₂O 9.769.74 9.75 9.58 9.41 9.74 9.72 9.58 wt % Y₂O₃ 1.84 3.54 1.81 (0.52)(1.01) (0.51) La₂O₃ V₂O₅ Ta₂O₅ WO₃ Nb₂O₅ CuO 0.35 0.35 0.37 0.34 0.360.37 0.38 0.37 SnO 0.10 0.12 0.10 0.11 0.11 0.12 0.11 0.12 F 0.79 0.790.79 0.78 0.79 0.79 0.78 0.79 Cl 0.40 0.46 0.49 0.53 0.56 0.43 0.42 0.47Total 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 VickersAvg. 420 418 408 405 403 411 415 414 Hv Max. 427 428 415 413 412 427 423423 Min. 413 400 401 398 394 400 409 403 Knoop Avg. 495 482 451 455 459470 467 463 Hk Max. 501 493 459 465 468 479 476 464 Min. 482 472 446 449450 462 457 441 Near-field extinction 44.0 45.5 45.7 45.9 46.1 45.1 44.845.5 ratio*/dB Shift in polarization 0.150 0.135 0.129 0.121 0.108 0.1360.142 0.131 axis/° Note: Values in parentheses in Y₂O₃, La₂O₃, V₂O₅,Ta₂O₅, WO₃, and Nb₂O₅ fields indicate contents in terms of mol %.Near-field extinction ratios indicate values measured at a measurementdistance of 15 mm.

TABLE 4 Comparative Example Example No. 9 No. 10 No. 11 No. 12 No. 13No. 1 No. 2 Raw material SiO₂ 59.43 60.23 58.82 59.92 58.79 60.92 56.43composition Al₂O₃ 7.89 7.99 7.82 7.96 7.94 8.17 10.65 in glass B₂O₃18.70 18.99 18.58 18.91 18.87 19.40 19.14 substrate/ Na₂O 9.50 9.62 9.409.58 9.55 9.94 10.99 wt % Y₂O₃ La₂O₃ 2.64 (0.52) V₂O₅ 1.41 (0.49) Ta₂O₅3.62 (0.53) WO₃ 1.86 (0.51) Nb₂O₅ 2.08 (0.50) CuO 0.37 0.35 0.36 0.380.35 0.37 0.79 SnO 0.10 0.11 0.11 0.10 0.11 0.10 0.11 F 0.79 0.78 0.790.78 0.79 0.76 0.75 Cl 0.51 0.52 0.51 0.52 0.52 0.34 1.14 Total 100.00100.00 100.00 100.00 100.00 100.00 100.00 Vickers Avg. 408 410 405 406408 428 325 Hv Max. 414 417 416 415 416 439 333 Min. 402 402 399 400 402415 318 Knoop Avg. 465 470 465 470 463 504 365 Hk Max. 472 480 473 478470 509 372 Min. 458 463 458 463 455 497 356 Near-field extinction 45.845.8 45.8 45.8 45.8 40.5 37.5 ratio*/dB Shift in polarization 0.1250.124 0.124 0.123 0.123 0.182 0.153 axis/° Note: Values in parenthesesin Y₂O₃, La₂O₃, V₂O₅, Ta₂O₅, WO₃, and Nb₂O₅ fields indicate contents interms of mol %. Near-field extinction ratios indicate values measured ata measurement distance of 15 mm.

Each of the glass blocks according to Example Nos. 1 to 13 was cut intoa 120×280×40 mm size and cast into a ceramic mold. Then, the glasspieces were heat-treated for 8 hours at 690° C. to produce a glasscontaining copper chloride particles in the above-described glass block.This glass was processed to produce a 100×250×4.5 mm (thickness)slab-shaped glass preform with both sides optically polished.

The surface of this glass preform was etched by a thickness of 0.05 to0.1 mm in an etching liquid containing hydrofluoric acid, and then theglass preform was stretched with a drawing apparatus. Drawing wasperformed while the drawing temperature (stretching temperature) wasbeing controlled within a range of 660° C. to 680° C. (glass viscosityof 7×106 to 3×106 Pa·S) so that a tensile force of 24.0 to 24.5 MPa wasapplied, as calculated from the cross-sectional area of the drawn sheet.

The produced drawn sheet was polished to a size of 70 mm in length 0.2mm in thickness, reduced in a 1-atm hydrogen gas atmosphere at 435° C.for seven hours, and cut into a 15 mm square in a direction parallelwith the stretching direction to produce a polarizing glass.

The extinction ratio was measured using an LD light source with awavelength of 1.55 μm at a distance of 15 mm between the polarizingglass and the power meter serving as a detector in the measuring systemshown in FIG. 1. A Glan-Thompson prism with an extinction ratio of 65 dBwas used. The diameter of the linearly polarized beam incident upon thepolarizing glass was about 1 mm. The power meter used was the “AQ-2125manufactured by Ando” with a photodetector 5 mm in diameter. Theobtained extinction ratios (dB) are shown in Tables 1 and 2.Furthermore, assuming that the angle that gives the minimum amount oftransmission light (angle that gives the maximum extinction ratio) atthe center point in a 15-mm square region is 0, the angles that give theminimum amounts of transmission light were measured, using an LD lightsource with a wavelength of 1.55 μm, at eight points including the twopoints 5 mm away from the center point in a direction orthogonal to thestretching direction, the two points 5 mm away from the center point inthe stretching direction, and the four points 5 mm away from the centerpoint in the diagonal directions; the absolute values of angle shiftsfrom the reference angle at the center point were obtained to measurethe amounts of shift in the polarization axis; and the maximum of theshifts in the polarization axis at those measurement points was enteredin Tables 1 and 2.

FIG. 7 shows the relationship between the amount of Cl in the glasssubstrate and the near-field extinction ratio (at a measurement distanceof 15 mm). This figure shows that the near-field extinction ratio slowlyincrease as the amount of Cl in the glass substrate increases and startsto decrease after reaching its maximum.

The extinction ratios of the polarizing glasses obtained from theglasses according to Example Nos. 1 and 3 were measured using a 1.55 μmLD light source at measurement distances of 15, 30, 50, 70, 100, 150,200, 250, and 300 mm in that order. The results are shown in FIG. 5.

In addition, as shown in FIG. 3, a polarizing glass 1 measuring 15 mmsquare produced from the glass according to Example No. 1; a polarizingglass 2 that was produced by polishing the glass according to ExampleNo. 1 to 70 mm in length by 0.2 mm in thickness and then cutting thereduced sheet into a 15-mm square, at an angle of 45°, to the stretchingdirection; a commercially available magnetic garnet film 3 (manufacturedby Seikoh Giken Co., Ltd.) formed by a liquid phase epitaxy method; andSm—Co-based magnets 4 were assembled so that the polarization axes ofthe two polarizing glasses are tilted by 45° and were then bonded withan adhesive. The thickness of each polarizing glass was 0.2 mm, and thethickness of the magnetic garnet film was 0.4 mm. Furthermore, a quartzsingle mode fiber 5 was attached to the polarizing glass 2 with anadhesive to prototype a pigtail optical isolator for a wavelength of1.55 μm. The isolation of this pigtail optical isolator for a wavelengthof 1.55 μm was 30 dB.

The isolation of the 1.55 μm pigtail optical isolator produced from theglass according to Example No. 3 in the same manner was 32 dB.

Comparative Example No. 1 in a Case where Cu Particles are Contained

A Comparative Example of a glass containing Cu particles will now bedescribed.

A batch of raw materials was prepared to produce the compositionaccording to Comparative Example No. 1 in Table 2 (same as in ExampleNo. 1). Then, a 5-liter platinum crucible was placed in a cubic,front-doored, 800×800×800 mm-capacity melting furnace provided with a450 mm (vertical)×250 mm (horizontal) opening and two vents 60 mm indiameter at diagonal positions 300 mm away from the stirring rod on theceiling thereof, and the batch of raw materials was delivered throughthe front door. The other conditions for producing a glass block werethe same as those in Example No. 1.

The Vickers hardness and the Knoop hardness number were measured in theexactly same manner as in Example Nos. 1 to 13 to find the components inthe glass substrate by quantitatively analyzing the glass. Furthermore,a polarizing glass was produced in the same manner as in the Examples tomeasure the near-field extinction ratio and the shift in thepolarization axis. The obtained results are shown in Comparative ExampleNo. 1 of Tables 2 and 4.

At this time, the Cl content was measured as 0.34 wt %, which is lowerthan 0.40 wt % measured in the case of a highly hermetic melting furnacein Example No. 1. As a result of the low Cl content, the Vickershardness was 428, which is higher than 420 in Example No. 1.Furthermore, the Knoop hardness number was 504, which is higher than 495in Example No. 1.

The near-field extinction ratio at a measurement distance of 15 mm wasmeasured as 40.5 dB using an LD light source for a wavelength of 1.55μm. This value is lower than 44.0 dB in Example No. 1. Furthermore, theshift in the polarization axis was measured as 0.182°, which is largerthan 0.150° in Example No. 1.

For the above-described polarizing glass produced from the glass servingas a matrix according to Comparative Example No. 1, the extinctionratios were measured in the same manner as in the Examples using a 1.55μm LD light source at measurement distances of 15, 30, 50, 70, 100, 150,200, 250, and 300 mm in that order. The results are shown in FIG. 5together with the results in the Examples.

The extinction ratios of the polarizing glass produced from the glassaccording to the Comparative Example were below those of the Examples atall measurement distances of 15 mm to 300 mm. The extinction ratio was56.2 dB at a measurement distance of 300 mm.

A pigtail optical isolator for a wavelength of 1.55 μm was produced inthe same manner as in the Examples using two sheets of polarizing glassproduced in this manner. The isolation of the pigtail optical isolatorfor a wavelength of 1.55 μm that used the polarizing glass produced fromthe glass serving as a matrix according to this Comparative Example was28 dB.

Comparative Example No. 2 in a Case where Cu Particles are Contained

With an increased amount of replacement from Na₂O to NaCl, a batch ofraw materials was prepared so as to achieve the composition according toComparative Example No. 2 in Table 2 and melted in a 5-liter platinumcrucible covered with a platinum lid placed in a cubic, highly hermetic800×800×800 mm-capacity melting furnace without a front door. The otherconditions for producing a glass block were the same as those in ExampleNo. 1 etc.

The Vickers hardness and the Knoop hardness number were measured in theexactly same manner as in Comparative Example No. 1 to find thepercentages of the components contained in the glass substrate.Furthermore, polarizing glass was produced in the same manner as in theExamples to measure the near-field extinction ratio and the shift in thepolarization axis. The obtained results are shown in Comparative ExampleNo. 2 of Tables 2 and 4.

At this time, the Cl content was measured as 1.14 wt %, which is higherthan 0.85 wt %, the upper limit of the amount of Cl according to thisembodiment. As a result of the high Cl content, the Vickers hardness was325, which is smaller than the lower limit 360 of the Vickers hardness,and the Knoop hardness number was 365, which is lower than 400, thelower limit of the Knoop hardness number.

The near-field extinction ratio at a measurement distance of 15 mm wasmeasured as 37.5 dB using an LD light source for a wavelength of 1.55μm. This value is lower than the target 44.0 dB. In addition, the shiftin the polarization axis was measured as 0.153°, which is slightlylarger than the target shift of 0.150°.

A pigtail optical isolator for a wavelength of 1.55 μm was produced inthe same manner as in the Examples using two sheets of polarizing glassproduced in this manner. The isolation of the pigtail optical isolatorfor a wavelength of 1.55 μm that used the polarizing glass produced fromthe glass serving as a matrix according to this Comparative Example No.2 was 24 dB.

Examples in a Case where Ag Particles are Contained

Examples of glasses containing Ag particles will be described.

Glasses with the composition according to Example Nos. 14 to 19 in Table5 are examples of glasses containing components Y₂O₃, La₂O₃, V₂O₅,Ta₂O₅, WO₃, and Nb₂O₅. Each batch of raw materials was prepared so thatthe composition according to Example Nos. 14 to 19 in Table 5 wasachieved. Raw materials were delivered in nine batches in three hours at1420° C. into a 5-liter platinum crucible placed in the same meltingfurnace as used in Example Nos. 1 to 13, that is, a cubic, highlyhermetic 800×800×800 mm-capacity melting furnace without a front door ora vent. Subsequently, the raw materials were left at rest for 30 minutesand melted. Thereafter, the temperature was increased to 1490° C., andthe raw materials were left at rest for 30 minutes. Then, the rawmaterials were stirred with a crank-type stirring rod at a rotationalspeed of 80 rpm for 30 minutes and left at rest for 30 minutes. This wasrepeated twice. Then, after stirring for a third time for 30 minutes,the raw materials were left at rest for two hours to remove air bubblesin the glass melt, and then the temperature was decreased to 1450° C. in30 minutes. Then, the raw materials were subjected to finish stirringfor 30 minutes at a rotational speed of 50 rpm, poured into a castingmold, and cooled slowly at 520° C. to produce a glass block.

The Vickers hardnesses and the Knoop hardness numbers of the producedglasses according to Example Nos. 14 to 19 were measured in the exactlysame manner as in Example No. 1 etc. to find the components in the glasssubstrate by quantitatively analyzing the glasses. Furthermore,polarizing glass was produced in the same manner as in Example No. 1etc. to measure the near-field extinction ratios and the shifts in thepolarization axes. The obtained results are shown in Example Nos. 14 to19 of Tables 5 and 6.

The Cl contents of the glasses according to Example Nos. 14 to 19 towhich Y₂O₃ and other components were added were 0.50 to 0.58 wt %. It isknown that as a result of Y₂O₃ and other components being added, theamounts of Cl contained in the glasses increased, compared withComparative Example No. 3 described below.

As the Cl contents increased, the Vickers hardnesses decreased from 428to 412 through 416, like the polarizing glass containing Cu particles.Furthermore, like the Vickers hardnesses, the Knoop hardness numbersdecreased from 502 to 465 through 470.

In addition, the near-field extinction ratios measured at a measurementdistance of 15 mm using an LD light source for a wavelength of 1.55 μmwere 45.2 to 45.7 dB.

TABLE 5 Comparative Example Example No. 14 No. 15 No. 16 No. 17 No. 18No. 19 No. 3 Raw material SiO₂ 56.56 56.19 56.75 55.68 56.54 56.43 57.56composition Al₂O₃ 1.70 1.69 1.71 1.67 1.70 1.70 1.73 in glass B₂O₃ 18.7118.52 18.73 18.40 18.69 18.63 19.01 substrate/ Li₂O 1.84 1.83 1.85 1.811.84 1.83 1.87 wt % KCl 0.87 0.86 0.87 0.85 0.87 0.87 0.89 K₂O 7.80 7.737.83 7.67 7.80 7.78 7.90 BaO 3.21 3.19 3.23 3.16 3.20 3.19 3.26 TiO₂1.55 1.54 1.56 1.53 1.55 1.55 1.58 ZrO₂ 5.62 5.59 5.64 5.55 5.62 5.605.71 Y₂O₃ 1.65 (0.49) La₂O₃ 2.37 (0.50) V₂O₅ 1.34 (0.50) Ta₂O₅ 3.19(0.50) WO₃ 1.70 (0.50) Nb₂O₅ 1.94 (0.49) Subtotal 100.00 100.00 100.00100.00 100.00 100.00 100.00 AgCl 0.4823 0.4823 0.4823 0.4823 0.48230.4823 0.4823 Total 100.4823 100.4823 100.4823 100.4823 100.4823100.4823 100.4823 Cl content/wt % 0.58 0.56 0.50 0.54 0.52 0.53 0.32Vickers Avg. 412 414 415 413 416 414 428 Hv Max. 420 421 422 420 424 420439 Min. 405 407 407 409 407 406 415 Knoop Avg. 465 467 469 466 470 468502 Hk Max. 476 477 480 473 479 477 509 Min. 458 459 461 458 462 458 497Near-field extinction 45.7 45.5 45.3 45.5 45.2 45.3 39.6 ratio*/dB Shiftin polarization 0.129 0.132 0.135 0.133 0.137 0.134 0.155 axis/° Note:Values in parentheses in Y₂O₃, La₂O₃, V₂O₅, Ta₂O₅, WO₃, and Nb₂O₅ fieldsindicate contents in terms of mol %. Near-field extinction ratiosindicate values measured at a measurement distance of 15 mm.

TABLE 6 Comparative Example Example No. 14 No. 15 No. 16 No. 17 No. 18No. 19 No. 3 Raw material SiO₂ 57.67 57.27 57.97 56.75 57.72 57.56 58.87composition Al₂O₃ 1.67 1.66 1.68 1.64 1.67 1.67 1.70 in glass B₂O₃ 17.6917.56 17.77 17.39 17.70 17.65 18.05 substrate/ Li₂O 1.77 1.76 1.78 1.741.77 1.76 1.81 wt % K₂O 8.05 7.99 8.09 7.92 8.06 8.04 8.22 BaO 3.24 3.223.27 3.19 3.24 3.23 3.31 TiO₂ 1.57 1.56 1.58 1.55 1.57 1.57 1.60 ZrO₂5.70 5.66 5.73 5.61 5.71 5.68 5.82 Y₂O₃ 1.78 (0.53) La₂O₃ 2.46 (0.51)V₂O₅ 1.31 (0.48) Ta₂O₅ 3.37 (0.52) WO₃ 1.73 (0.50) Nb₂O₅ 2.01 (0.51) Ag0.29 0.29 0.31 0.30 0.31 0.29 0.30 Cl 0.58 0.56 0.50 0.54 0.52 0.53 0.32Total 100.00 100.00 100.00 100.00 100.00 100.00 100.00 Vickers Avg. 412414 415 413 416 414 428 Hv Max. 420 421 422 420 424 420 439 Min. 405 407407 409 407 406 415 Knoop Avg. 465 467 469 466 470 468 502 Hk Max. 476477 480 473 479 477 509 Min. 458 459 461 458 462 458 497 Near-fieldextinction 45.7 45.5 45.3 45.5 45.2 45.3 39.6 ratio*/dB Shift inpolarization 0.129 0.132 0.135 0.133 0.137 0.134 0.155 axis/° Note:Values in parentheses in Y₂O₃, La₂O₃, V₂O₅, Ta₂O₅, WO₃, and Nb₂O₅ fieldsindicate contents in terms of mol %. Near-field extinction ratiosindicate values measured at a measurement distance of 15 mm.

Comparative Example in a Case where Ag Particles are Contained

A Comparative Example of a glass containing Ag particles will bedescribed.

A batch of raw materials was prepared so that the composition accordingto Comparative Example No. 3 in Table 5 was achieved. This glass wasproduced as a glass block in the same manner as in Example No. 14 etc.,except that the melting temperature was 20° C. lower than that ofExample Nos. 14 to 19 in all schedules and the slow cooling temperaturewas 510° C.

The Vickers hardness and the Knoop hardness number were measured in theexactly same manner as in Example No. 14 etc. Furthermore, the glass wasquantitatively analyzed to find components contained in the glasssubstrate. Moreover, a polarizing glass was produced in the same manneras in Example No. 14 etc. to measure the near-field extinction ratio andthe shift in the polarization axis. The obtained results are shown inComparative Example No. 3 of Tables 5 and 6.

At this time, the Cl content was measured as 0.32 wt %, the Vickershardness was 428, and the Knoop hardness number was 502. The near-fieldextinction ratio at a measurement distance of 15 mm was measured as 39.6dB using an LD light source for a wavelength of 1.55 μm, and the shiftin the polarization axis was measured as 0.155°.

A pigtail optical isolator for a wavelength of 1.55 μm was produced inthe same manner as in Example No. 14 etc. using two plates of polarizingglass produced in this manner. The isolation of the pigtail opticalisolator for a wavelength of 1.55 μm that used the polarizing glassesproduced from the glass serving as a matrix according to thisComparative Example No. 3 was 26 dB, which is below the specificationvalue of 30 dB.

Comparison Between Examples and Comparative Example in a Case where AgParticles are Contained

The Cl content in the glass according to Comparative Example No. 3 towhich Y₂O₃ or other components were not added was 0.32 wt %, whereas theCl contents in the glasses according to Example Nos. 14 to 19 to whichY₂O₃ and other components were added were 0.50 to 0.58 wt %. It is knownthat adding Y₂O₃ and other components increased the amount of Cl in theglass.

The extinction ratios in the Examples were higher than 39.6 dB inComparative Example No. 3 in which Y₂O₃ or other components were notadded. The shifts in the optical axis were measured as 0.129 to 0.137°,which are smaller than 0.155° in Comparative Example No. 3 and are belowthe target value 0.150°.

Also in Example Nos. 14 to 19, a pigtail optical isolator for awavelength of 1.55 μm was produced in the same manner as in ComparativeExample No. 3 using two plates of polarizing glass obtained in thismanner. The isolations of the pigtail optical isolators for a wavelengthof 1.55 μm that used the polarizing glass produced from the glassserving as a matrix according to Example Nos. 14 to 19 were 31 to 32 dB,which are 5 to 6 dB higher than that measured when the glass accordingto Comparative Example No. 3 in which components were not added wasused. Consequently, the pigtail optical isolators according to ExampleNos. 14 to 19 had sufficient performance as pigtail optical isolators.

Form the description so far, the following points have been clarified asa result of comparison between the Examples and the ComparativeExamples. If the amount of Cl contained in the glass was 0.40 to 0.85 wt%, then a glass with a Vickers hardness of 360 to 420 and a Knoophardness number of 400 to 495 was produced. In these polarizing glassesproduced from the glasses serving as a matrix according to the presentinvention, the near-field extinction ratios measured at a measurementdistance of 15 mm increased to 44 dB or more, and the amounts of shiftin the polarization axis at positions 5 mm away from the center were0.15° or less, demonstrating that a polarizing glass suitable for apigtail optical isolator was produced.

The present invention can be applied to a polarizer used in, forexample, a compact optical isolator for optical communication etc.; anoptical switch composed of a liquid crystal, an electro-optic crystal, aFaraday rotator, and so forth; or an electro-magnetic sensor.

1. A polarizing glass comprising anisotropically shaped metal particlesoriented and dispersed in a glass substrate, wherein the glass substrateis a glass selected from the group consisting of silicate glass, borateglass, and borosilicate glass; the glass substrate contains at least onecomponent selected from the group consisting of Y₂O₃, La₂O₃, V₂O₅,Ta₂O₅, WO₃, and Nb₂O₅; the content of each of the selected componentsranges from 0.05 to 4 mole percent, or the total content of thecomponents is 6 mole percent or less if a plurality of components areselected; and the Cl content in the glass substrate ranges from 0.47 to0.58 wt % relative to the entire glass substrate.
 2. The polarizingglass according to claim 1, wherein the anisotropically shaped metalparticles are anisotropically shaped copper particles, and raw materialsof the glass substrate are composed of, in terms of wt %: SiO₂: 48-65,B₂O₃: 13-33, Al₂O₃: 6-13, AlF₃: 0-5, alkali metal oxide: 7-17, alkalimetal chloride: 0-5, alkali earth metal oxide: 0-5, copper oxide andcopper halide: 0.3-2.5, SnO: 0-0.6, and As₂O₃: 0-5.
 3. The polarizingglass according to claim 1, wherein the anisotropically shaped metalparticles are anisotropically shaped copper particles, and the glasssubstrate is composed of, in terms of wt % relative to the entire glasssubstrate: SiO₂: 55-63, B₂O₃: 16-22, Al₂O₃: 7-10, alkali metal oxide:8-12, alkali earth metal oxide: 0-3, copper oxide: 0.3-1.0, SnO: 0-0.3,As₂O₃: 0-3, and F: 0.5-1.0.
 4. The polarizing glass according to claim1; wherein an extinction ratio at a measurement distance of 15 mm is45.2 dB or more in response to at least one of light in a wavelengthband with a center wavelength of 1.31 μm and light in a wavelength bandwith a center wavelength of 1.55 μm.
 5. The polarizing glass accordingto claim 1; wherein the maximum of the absolute values of the amounts ofshift in polarization axis at sites 5 mm away from a center point on aprinciple surface is 0.137° or less.
 6. An optical isolator comprisingthe polarizing glass according to claim
 1. 7. An optical isolatorcomprising a Faraday rotating element and at least one polarizer,wherein the polarizer includes the polarizing glass according toclaim
 1. 8. A process of producing a polarizing glass containinganisotropically shaped metal particles oriented and dispersed in a glasssubstrate by stretching a glass preform, the process comprising: meltingraw materials of the glass substrate in an airtight melting furnace; andreplacing at least one of the alkali oxide components in the rawmaterials of the glass substrate with an alkali chloride component,wherein the glass substrate is a glass selected from the groupconsisting of silicate glass, borate glass, and borosilicate glass; theglass substrate contains at least one component selected from the groupconsisting of Y₂O₃, La₂O₃, V₂O₅, Ta₂O₅, WO₃, and Nb₂O₅; the content ofeach of the selected components ranges from 0.05 to 4 mole percent, orthe total content of the components is 6 mole percent or less if aplurality of components are selected; and the Cl content in the glasssubstrate ranges from 0.47 to 0.58 wt % relative to the entire glasssubstrate.
 9. The process of producing a polarizing glass according toclaim 8, wherein the anisotropically shaped metal particles are copperparticles, and raw materials of the glass substrate are melted, the rawmaterials being composed of, in terms of wt %: SiO₂: 48-65, B₂O₃: 13-33,Al₂O₃: 6-13, AlF₃: 0-5, alkali metal oxide: 7-17, alkali metal chloride:0-5, alkali earth metal oxide: 0-5, copper oxide and copper halide:0.3-2.5, SnO: 0-0.6, and As₂O₃: 0-5.